The present invention relates to oscillating hook type sewing machines, and more particularly to improvements in sewing machines wherein the shuttle hook oscillates in a substantially vertical plane.
Oscillating hook type sewing machines exhibit the advantage that the needle thread loop is not twisted during training around the hook and bobbin case and that the thread can readily pass between the hook and the shuttle driver finger, as a result of mere reversal in the direction of movement of the hook and driver finger, without any specially designed auxiliary equipment. As a rule, such types of hooks are not sensitive to changes in the tension of the thread and can assist in the making of eye-pleasing uniform stitches.
Friction between the limbs of the loop which is formed by needle thread often entails the making of slack stitches. This can take place in all kinds of sewing machines including those employing heretofore known oscillating type hooks. The magnitude of friction between the limbs of a loop depends on a plurality of parameters including the diameters and types of threads, the type of stitching, the length of loops, the width of stitches, the speed of the sewing machine and others. The just discussed friction can result in the making of non-uniform stitches.
For example, the loop which is formed by the needle thread in an oscillating hook type sewing machine must completely surround the hook, the bobbin case and the supply of bobbin thread in the case. The loop is cast off shortly or immediately before the hook changes the direction of its movement, and the thus released loop is lifted by the takeup lever so that its bight is moved against the underside of the work. As a rule, upward movement of the takeup lever entails an upward movement of the front limb of the needle thread loop, i.e., of that limb which extends through the eye of the needle. The rear limb is more or less passive. That portion of the bobbin thread which extends from the bobbin case to the underside of the work passes between the two limbs of the loop which is formed by the needle thread and normally does not interfere with a predictable reduction of the size of the needle thread loop. However, when the machine is set to make long and/or wide stitches, the needle penetrates behind that portion of the bobbin thread which extends from the bobbin case to the work during transition from a right downward stroke to a left downward stroke. Consequently, the needle thread is looped around the bobbin thread subsequent to castoff from the hook which entails the development of additional friction in the region of the front limb of the needle thread loop. This, in turn, entails a more rapid upward movement of the rear limb of the needle thread loop. The just described mode of operation does not appreciably affect the quality and/or appearance of the stitches when the sewing machine is operated at a medium speed or at an elevated speed because the making of stitches takes place at frequent intervals and the inertia of the rear limb is too pronounced so that it cannot react to the development of additional friction with the front limb. However, the situation is different when the sewing machine is operated at less than average speed or at a low speed. At such time, the upper and lower threads are in longer-lasting frictional engagement with each other because the speed at which the size of the needle thread loop is reduced is relatively low and the interval of frictional engagement between the two slowly moving threads is much longer. In other words, that component of static friction which causes the two threads to adhere to each other is more pronounced than the component of sliding friction. The upward movement of the rear limb of the needle thread loop is too rapid with the result that the entire needle thread is not drawn all the way into the work and the work is formed with so-called slack or loose stitches. Since the making of slack stitches takes place at random, they greatly affect the appearance of the product.
Attempts to avoid the making of slack stitches in other types of sewing machines include the utilization of retainers of the type disclosed in U.S. Pat. No. 4,095,539 which describes and shows a sewing machine with a rotary shuttle. This patent proposes to use a so-called work limb retainer which engages the loop during the normal loop taker cycle and discharges the thread after the thread has completed its passage around the loop taker. A somewhat similar proposal is described in German Utility Model No. 70 16 286.
The incorporation of features which lessen or eliminate the aforediscussed problems in rotary shuttle type sewing machines into an oscillating shuttle hook type sewing machine is impossible because of the entirely different nature and mode of operation of shuttles in such machines.
German Offenlegungsschrift No. 33 42 770 discloses an oscillating hook type sewing machine wherein the shuttle hook driver carries a device for braking the rear limb of the needle thread loop. A drawback of this proposal is that the braking device is complex, expensive and bulky. Moreover, the nature of the braking device which is disclosed in the Offenlegungsschrift is such that each of a short or long series of braking devices produces a different braking force so that the braking action must be individually adjusted in each and every sewing machine which employs the proposed braking device. This contributes significantly to the initial and maintenance cost of the sewing machine.